Did you test the ECT when it was hard to start, or was this after it sat and heat soaked? Did you try opening the throttle a bit while you cranked? I don't remember if you described this problem here.

my boat came with and operator manual calls out a 143 so that's what I use... no issues what so ever. I even tried opening the throttle a bit - discussed the issue 3 seasons ago - been running a 143 without issue since 2011 season.

Regarding the ECT - the temp after heat soak was over 170 - it was at that temp that the engine would not start. when it dropped below 160 it would fire up.

__________________...A bad day water skiing still beats a good day at work...1995 Pro Star 205....

I don’t know what things might have been like for others or in times past, but my experience with Mastercraft in the six months I have owned a Mastercraft is they have helped with a replacement trailer plate and hull color lookup based on hull id, but they beg off of engine questions and send me to Indmar for that. That’s what they did when I called them months back about the thermostat issue.

I did call Indmar back then. I had forgotten until you mentioned it just now, but they did ask for the engine serial number before answering what thermostat my engine took. I remember because I remember the guy remarking it was unusual I knew what the first two letters stood for (think I learned from a post of yours). So the Indmar answer, based on my engine serial number, is my 350 TBI started its life with a 140 something thermostat. This jibes with what Engine Nut posted here in 2007 about TBIs generally (link in my previous post).

Having established that, I would think the issue would be, what is the best thinking now on what should be put on such an engine with twenty years of additional information and any relevant local factors such as ambient temperature.

I’m in central Florida, equidistant between the coasts operating in a county that has been known to compete in the summer with Death Valley, California for the hottest temperature nationally. My primary lake is mostly fed by rainwater directly and as runoff and gets little to no cool spring water. We have much cooler spring fed rivers in this county, but I don’t use them with my boat. Guess that means you think I would be at higher risk of a problem by moving to a 160 thermostat, but none of this deters DIM from recommending a 160 thermostat.

I wonder what are the implications of sticking a 160 thermostat in an engine such as mine that was otherwise set up for a lower temperature thermostat.

Have you ever removed a radiator cap when the engine is hot? Of course not- it'll spew coolant all over you. Why does this happen? Because cars and trucks operate at temperatures that approach the boiling point of water and many use a thermostat that opens at 180 degrees, or more. The engine in your boat is really no different from the 5.7L TBI that's in millions of cars and trucks and it will work just fine with a 160 degree thermostat, as long as it's not starved for raw water and/or fuel. A lot of the difference in the calibration and setup is due to the fact that ski boats often go from idle to WOT, or close to it, in a second. Cars and trucks aren't operated this way in most cases. That means more fuel needs to be delivered, is order to make it survive.

20 degrees isn't enough to cause catastrophic failure unless certain conditions are met. Detonation (knocking) occurs more when the air is hot and dry and high operating temperature causes the ECM to reduce the amount of fuel delivered and if it sees extremely high temperature, it can be too lean for running at high RPM. At idle or low speed, it's fine, though. This is the reason a 143 degree thermostat can be recommended. The ECM's programming isn't as finely tuned as it is in a car or truck, but it does take the operating temperature into account when opening the injectors. Extremely warm raw water makes heat soak problems more likely, but basically, this means nothing when the engine is off- the thermostat will open completely regardless of which thermostat you have and once the raw water supply has returned, the engine will run normally. If it stumbles immediately after re-starting and the temperature gauge shows more than 200 degrees, it means you should wait before hitting the throttle hard. If you have a temperature gauge in your car or truck, look at it on a hot day before you re-start it by turning the key ON, but don't crank it. It will show temperatures in the 220-240 range.

As I posted, letting the engine cool down a bit before shutting it off is the best way to avoid heat soak issues. If it's extremely hot, open the motor box a bit and/or turn the blower on, so fresh air can remove some of the hot air. IIRC, they had calibration files specifically for high temperature locations, just as they did for high altitude locations and these were created after going to those places with MC boats, operated under the same conditions that created the problems. These were available to dealers, for reprogramming without requiring the ECM to be sent to MC or Indmar.

One other thing that poses a risk to a hot engine that has been allowed to heat soak- if it's using fresh water to cool, an extremely hot engine receiving extremely cold water is something that should be avoided, although you would never see this where you are, unless a cold spring feeds the body of water.

Either will work- if you smell raw gas in the exhaust and see soot on the transom with a 140 degree thermostat, you could change to the 160. You would also see black, heavy soot on the spark plugs and that's a good indication of excessively rich fuel mixture, which doesn't help anything and can cause other problems, like unburned fuel getting into the oil, because it bypasses the rings. This dilutes the oil and reduces its lubricity and that causes wear.

FWIW, I had a '90 Chevy Silverado with the same engine. The odometer showed over 140K in 2001 and I would estimate the total mileage when sold it a little more than a year ago was over 300K (the odometer didn't work when I got it). My friend and I changed the valve seals and the valve covers were almost spotless. That's as far as I went into the engine because it didn't need more than that. It uses a 194 degree thermostat. When I sold it, it still ran great.

I'm not telling you to treat it like a farm animal, but these are extremely durable and reliable engines- that's why they were used by GM for 50 years. Considering the number of overheated engines that are caused by neglected impellers and oil coolers out there, the number of failures is very small. I saw quite a few boats with melted exhaust hoses, flanges and flappers and they just kept going after those were replaced.

my boat came with and operator manual calls out a 143 so that's what I use... no issues what so ever. I even tried opening the throttle a bit - discussed the issue 3 seasons ago - been running a 143 without issue since 2011 season.

Regarding the ECT - the temp after heat soak was over 170 - it was at that temp that the engine would not start. when it dropped below 160 it would fire up.

If it didn't start above 170, did you ever open the throttle a bit and find that it started easily? That indicates the ECT was/is bad. If that tells the ECM that it's cold, it will deliver too much gas and opening the throttle corrects the fuel/air mixture. My van shows well over 200 degrees in Summer when I turn the key ON and it starts fine. The ECM doesn't react to overheat until 200 degrees and your temperature gauge means nothing WRT what the ECM receives as temperature information. Did you ever test the ECM's coolant sensor?

If it didn't start above 170, did you ever open the throttle a bit and find that it started easily? That indicates the ECT was/is bad. If that tells the ECM that it's cold, it will deliver too much gas and opening the throttle corrects the fuel/air mixture. My van shows well over 200 degrees in Summer when I turn the key ON and it starts fine. The ECM doesn't react to overheat until 200 degrees and your temperature gauge means nothing WRT what the ECM receives as temperature information. Did you ever test the ECM's coolant sensor?

when the temp was up - the injectors were throwing very little gas. I bought a new ECT this winter - installing it this spring. I still have the 160 t-stat. I never smell raw gas anywhere - engine runs very well with current parts so I can't complain.

no never tested the ECM coolant sensor - when the called for t-stat(143) was installed and the motor ran fine again - I put the issue behind me. I changed 2 of the temp sensors in an old thread and did not know about the third one till this thread - so I guess the new sensor will allow the 160 operate as well as the 143. learned something new about the boat this week.

I was never corrected that the coolant switch in the t-stat housing with the one wire.

We discussed this issue in other threads and based on that I went to the route of returning to the 143 t-stat.

__________________...A bad day water skiing still beats a good day at work...1995 Pro Star 205....

when the temp was up - the injectors were throwing very little gas. I bought a new ECT this winter - installing it this spring. I still have the 160 t-stat. I never smell raw gas anywhere - engine runs very well with current parts so I can't complain.

no never tested the ECM coolant sensor - when the called for t-stat(143) was installed and the motor ran fine again - I put the issue behind me. I changed 2 of the temp sensors in an old thread and did not know about the third one till this thread - so I guess the new sensor will allow the 160 operate as well as the 143. learned something new about the boat this week.

I was never corrected that the coolant switch in the t-stat housing with the one wire.

We discussed this issue in other threads and based on that I went to the route of returning to the 143 t-stat.

The ECM only goes into RPM reduction when it thinks the ECT is reading 200°F, but it can't know if that's correct- it's just the resistance of the ECT that gives it this info. One thing to remember- water is a much better conductor of heat than air, so it may have actually been overheating when the gauge read 170. I'd be interested in knowing if something was blocking the raw water, the water temperature or if something else was causing the overheat. I have never seen a 160° thermostat cause an overheat when nothing else was affecting the cooling. It's supposed to open at 160° and if it was just sticking, that would definitely cause an overheat because the thermostat was bad, not because it was rated for 160°. Brackish water is a really bad culprit for this, as well as other corrosion problems- not that you would see this, but with the chemicals in some water, it's hard to know what will happen over time.

The ECM only goes into RPM reduction when it thinks the ECT is reading 200°F, but it can't know if that's correct- it's just the resistance of the ECT that gives it this info. One thing to remember- water is a much better conductor of heat than air, so it may have actually been overheating when the gauge read 170. I'd be interested in knowing if something was blocking the raw water, the water temperature or if something else was causing the overheat. I have never seen a 160° thermostat cause an overheat when nothing else was affecting the cooling. It's supposed to open at 160° and if it was just sticking, that would definitely cause an overheat because the thermostat was bad, not because it was rated for 160°. Brackish water is a really bad culprit for this, as well as other corrosion problems- not that you would see this, but with the chemicals in some water, it's hard to know what will happen over time.

One thing that made me very curious was what the heat soak was with the 143 t-stat. It was the same amount - 10-15 degrees over running temp. I bought an infrared hand held thermometer from harbor freight just for giggles.

I subscribed to this thread and will report back this season - I understand what you are saying - one of the reasons I bought a new ECT sensor - 18 year part - time to replace could be a number of circumstances.

I truly appreciate your insight and knowledge Jim

__________________...A bad day water skiing still beats a good day at work...1995 Pro Star 205....

Have you ever removed a radiator cap when the engine is hot? Of course not- it'll spew coolant all over you. Why does this happen? Because cars and trucks operate at temperatures that approach the boiling point of water and many use a thermostat that opens at 180 degrees, or more. The engine in your boat is really no different from the 5.7L TBI that's in millions of cars and trucks and it will work just fine with a 160 degree thermostat, as long as it's not starved for raw water and/or fuel. A lot of the difference in the calibration and setup is due to the fact that ski boats often go from idle to WOT, or close to it, in a second. Cars and trucks aren't operated this way in most cases. That means more fuel needs to be delivered, is order to make it survive.

20 degrees isn't enough to cause catastrophic failure unless certain conditions are met. Detonation (knocking) occurs more when the air is hot and dry and high operating temperature causes the ECM to reduce the amount of fuel delivered and if it sees extremely high temperature, it can be too lean for running at high RPM. At idle or low speed, it's fine, though. This is the reason a 143 degree thermostat can be recommended. The ECM's programming isn't as finely tuned as it is in a car or truck, but it does take the operating temperature into account when opening the injectors. Extremely warm raw water makes heat soak problems more likely, but basically, this means nothing when the engine is off- the thermostat will open completely regardless of which thermostat you have and once the raw water supply has returned, the engine will run normally. If it stumbles immediately after re-starting and the temperature gauge shows more than 200 degrees, it means you should wait before hitting the throttle hard. If you have a temperature gauge in your car or truck, look at it on a hot day before you re-start it by turning the key ON, but don't crank it. It will show temperatures in the 220-240 range.

As I posted, letting the engine cool down a bit before shutting it off is the best way to avoid heat soak issues. If it's extremely hot, open the motor box a bit and/or turn the blower on, so fresh air can remove some of the hot air. IIRC, they had calibration files specifically for high temperature locations, just as they did for high altitude locations and these were created after going to those places with MC boats, operated under the same conditions that created the problems. These were available to dealers, for reprogramming without requiring the ECM to be sent to MC or Indmar.

One other thing that poses a risk to a hot engine that has been allowed to heat soak- if it's using fresh water to cool, an extremely hot engine receiving extremely cold water is something that should be avoided, although you would never see this where you are, unless a cold spring feeds the body of water.

Either will work- if you smell raw gas in the exhaust and see soot on the transom with a 140 degree thermostat, you could change to the 160. You would also see black, heavy soot on the spark plugs and that's a good indication of excessively rich fuel mixture, which doesn't help anything and can cause other problems, like unburned fuel getting into the oil, because it bypasses the rings. This dilutes the oil and reduces its lubricity and that causes wear.

FWIW, I had a '90 Chevy Silverado with the same engine. The odometer showed over 140K in 2001 and I would estimate the total mileage when sold it a little more than a year ago was over 300K (the odometer didn't work when I got it). My friend and I changed the valve seals and the valve covers were almost spotless. That's as far as I went into the engine because it didn't need more than that. It uses a 194 degree thermostat. When I sold it, it still ran great.

I'm not telling you to treat it like a farm animal, but these are extremely durable and reliable engines- that's why they were used by GM for 50 years. Considering the number of overheated engines that are caused by neglected impellers and oil coolers out there, the number of failures is very small. I saw quite a few boats with melted exhaust hoses, flanges and flappers and they just kept going after those were replaced.

Okay, I hear you saying changing to the higher temperature thermostat isn’t likely to create a problem, but the supposed motivation for changing thermostats is improvement. Without commenting on their validity, because I’m not qualified, the arguments I’ve heard for changing to the higher temperature thermostat (160 degree) are internal components will settle into more comfortable dimensions, lubrication will improve, and fuel will atomize better (this last one is published on both DIM and Bakes websites).

If you had an Indmar 350 TBI that came out of the factory with a 142 degree thermostat and set up for it, changing the ECM and/or its programming was outside the scope of what you were doing, you were pulling skiers in a quite warm fresh water lake near sea level, you had both thermostats on hand and didn’t care about the cost in money or time to change the thermostat, and your goal was to have your engine run well and long into the future, what is the bottom line on which thermostat you would run in your boat ? A 142 degree thermostat or a 160 degree thermostat?

EDIT: And since you have stated elsewhere that boats sold in especially warm areas may have been set up differiently, let's add that the boat was registered in Tennessee at least as far back as 2002 and probably was sold new in Tennessee, not in Florida.

Okay, I hear you saying changing to the higher temperature thermostat isn’t likely to create a problem, but the supposed motivation for changing thermostats is improvement. Without commenting on their validity, because I’m not qualified, the arguments I’ve heard for changing to the higher temperature thermostat (160 degree) are internal components will settle into more comfortable dimensions, lubrication will improve, and fuel will atomize better (this last one is published on both DIM and Bakes websites).

If you had an Indmar 350 TBI that came out of the factory with a 142 degree thermostat and set up for it, changing the ECM and/or its programming was outside the scope of what you were doing, you were pulling skiers in a quite warm fresh water lake near sea level, you had both thermostats on hand and didn’t care about the cost in money or time to change the thermostat, and your goal was to have your engine run well and long into the future, what is the bottom line on which thermostat you would run in your boat ? A 142 degree thermostat or a 160 degree thermostat?

A warm engine is more efficient. If you see the temperature gauge reaching 200 degrees with either of these, shut it down. As far as performance, I doubt there would be much difference because the ECM's fuel table for various temperatures, so it will still run well, but it's not as exact as an engine with more sensors, like Intake Air temperature, MAF, O2, etc. These don't need to hit the same EPA numbers, but the injection system was installed to improve reliability and ease of starting. It's a lot more consistent than a carb.

A warm engine is more efficient. If you see the temperature gauge reaching 200 degrees with either of these, shut it down. As far as performance, I doubt there would be much difference because the ECM's fuel table for various temperatures, so it will still run well, but it's not as exact as an engine with more sensors, like Intake Air temperature, MAF, O2, etc. These don't need to hit the same EPA numbers, but the injection system was installed to improve reliability and ease of starting. It's a lot more consistent than a carb.

It sounds to me like your answer would be “160” if you had to give a three-digit answer, but you don’t have to and you didn’t.

Your reference to the temperature reported on the gauge brings to mind a concern I have about how accurately what is reported on the temperature gauge reflects reality.

And that’s with a gauge and sender within tolerance. Of course, the reported temperature could be even further off if the gauge or sensor were not within tolerance.

I’m 6 feet 3 inches tall. If someone asked how tall I was and I said I was between 5 feet 10 inches and 6 feet 8 inches tall, people might think I was being evasive, but if I were one of these temperature gauge/sensor setups on these boats, I would be answering within the designed tolerance.

It seems the temperature gauge is a pretty rough guide to what is going on. I’ve seen its data presented at times on the forum (not trying to point a finger at anyone) where temperature differences that could be nothing but measurement error have been treated as real differences.